Gas sensor, in particular a lambda sensor

ABSTRACT

The reference air channel of a gas sensor or a lambda probe having a laminate body produced by printing technology is provided. The laminate body is produced by printing a suitably structured layer onto a neighboring layer, for example, by screen printing.

BACKGROUND INFORMATION

[0001] The present invention relates to gas sensors, in particular lambda probes, having a body designed as a sintered ceramic laminate and therein a reference air channel arranged inside a layer of the laminate, an electric resistance heater situated or embedded in the laminate being provided on one side thereof and an electrode arrangement is provided on its other side having at least one reference electrode which is arranged inside a bordering wall of the reference air channel and is gas-permeable in at least some areas and also having a Nernst electrode which is acted upon by the gas to be sensed and is also designed to be gas-permeable in at least some areas and is separated from the reference electrode by a solid electrolyte layer that is conductive and permeable for ions, in particular oxygen ions.

[0002] Exhaust systems in modern internal combustion engines are usually provided with catalysts for catalytic decomposition of noxious exhaust gases, in particular in motor vehicles today. For good functioning of the catalysts, air and fuel must be supplied to the engine in a predetermined ratio. The engine controls provided for this purpose are connected at their input end to a lambda probe, the signals of which indicate the composition of the exhaust gases and thus make it possible for the engine control to regulate the ratio of fuel and combustion air in an optimum manner for the catalyst.

[0003] Two concepts are known in this regard.

[0004] With the one concept, a stoichiometric combustion is the goal, i.e., the quantity of oxygen in the combustion air corresponds exactly to the oxygen demand for complete combustion of the fuel supplied. Thus, the engine is not operated with an oxygen excess (λ>1) or an oxygen deficiency (λ<1). This type of operation may be characterized by λ=1 accordingly.

[0005] For this stoichiometric combustion, lambda probes that operate with a narrow band, the Nernst electrode being acted upon virtually directly by the exhaust gas, are sufficient for exhaust gas sensing.

[0006] The engine control here makes use of the effect that an electric voltage which is generated by diffusion of oxygen ions and is detectable between the reference electrode and the Nernst electrode undergoes a great change in value in the range of λ=1, and a signal which clearly indicates a deviation from the desired operation with stoichiometric combustion in the direction of operation with a deficiency of oxygen as well as in the direction of operation with an excess of oxygen is available accordingly.

[0007] Such sensors are described in German Patent Application 44 01 749 A1, for example.

[0008] In the other concept, operation of the combustion engine predominantly with an excess of oxygen in combustion is the goal, because this allows a significant reduction in fuel consumption. However, harmful nitrogen oxides formed in combustion with an excess of oxygen may be absorbed by storage catalysts in the exhaust line of an automotive engine only for a limited period of time. Before the storage capacity of these storage catalysts is depleted, operation of the engine must be switched briefly to combustion with an oxygen deficiency in order to be able to reduce the nitrogen oxides previously stored in the catalyst by incompletely burned fuel constituents now entering the exhaust gas line. The engine control must therefore be switched repeatedly and intermittently between a mode of operation that is predominant over a period of time in which the values of λ are above 1 and a relatively short-term mode of operation in which the values of λare less than 1.

[0009] Broadband lambda probes are necessary for such intermittent operation with greatly varying values of λ.

[0010] With such lambda probes, the Nernst electrode is arranged in a separate chamber which communicates with the exhaust gas stream via a diffusion zone in the body of the probe. In addition, an internal pump electrode, which is situated inside this chamber and may be connected electrically to the Nernst electrode and cooperates with an external pump electrode through a solid electrolyte layer, is exposed virtually directly to the exhaust gas stream. If an external electric voltage is applied between the two pump electrodes, both being designed to be gas-permeable in at least some areas, then an oxygen ionic current in a direction which depends on the polarity of the applied voltage and having an amperage which depends on the electric potential difference is generated between the pump electrodes, with the result that the diffusion stream of the exhaust gases into the diffusion chamber is controlled accordingly. The external electric voltage between the pump electrodes and the electric current occurring between the pump electrodes because of the oxygen ionic current are then adjusted by a regulator so that an electric voltage having a predetermined setpoint is always maintained between the reference electrode and the Nernst electrode. Thus the polarity and amperage of the electric current occurring between the pump electrodes constitute a signal which correlates with the composition of the exhaust gases and thus with the λ values.

[0011] Such probes are described in German Patent Application 37 44 206 A1, for example.

[0012] All the probes described previously must be heated during operation to generate an analyzable signal. Therefore, lambda probes and other gas sensors usually have an electric resistance heater, which in the case of a probe body formed by a laminate is situated on or between layers of the laminate.

ADVANTAGES OF THE INVENTION

[0013] According to the present invention, the reference air channel is designed in a structured layer or a layer arrangement produced by printing technology.

[0014] This offers the advantage that practically any desired shapes including small parts are possible for the reference air channel in comparison with the type of production customary in the past where the parts are punched out of a green ceramic film.

[0015] For example, the contours of the reference air channel may be adapted to the contours of the electric resistance heater, which are usually meandering, or in a top view of the layer planes they may be removed by an entrance hole for the exhaust gases passing through the probe body perpendicular to the layer planes.

[0016] In addition, it is also possible to divide the mouth of the reference air channel like a fan for the admission of the reference air and/or to position the layer or layer arrangement forming the reference air channel on the longitudinal edges, the layer or layers of the layer arrangement optionally being broken down into non-coherent parts without thereby increasing the manufacturing complexity.

[0017] Due to the smaller cross sections of the reference air channel which are possible with production by printing technology and due to the comparatively small height of this channel perpendicular to the planes of the layer, a good heat conducting connection is created between the parts of the probe body on both sides of the layer or layer arrangement accommodating the reference air channel, so that the thermal stresses occurring at the start of heating within the probe body remain low or particularly rapid heating of the probe body becomes possible.

[0018] Moreover it is possible to provide for the reference air channel to be filled with a porous mass, thus permitting an even better heat transfer between the laminate parts on both sides of the reference air channel.

[0019] Production of the reference air channel by printing technology is expediently accomplished in such a manner that a negative pattern of the reference air channel is printed on the side of a layer which carries the heater or sheaths the heater, this side facing the reference air channel, and/or on the side (of the solid electrolyte layer arranged between the reference electrode and the Nernst electrode) carrying the reference electrode, this pattern being printed using a pasty material that hardens under heat, in particular zirconium oxide paste.

[0020] In addition, it is also possible to provide for the positive shape of the reference air channel to be printed with a pasty material that burns up under heat or burns off, forming a porous structure. This guarantees particularly reliably that the reference air channel has a reproducible height perpendicular to the planes of the layer of the laminate.

[0021] In summary, it may thus be concluded that the present invention implements the general idea of producing the reference air channel by technical printing means to be able to produce virtually any desired filigree structures very reproducibly with a low manufacturing complexity.

DRAWINGS

[0022] Embodiments of the lambda probes according to the present invention are illustrated in the drawing and explained in greater detail below.

[0023]FIG. 1 shows a cross section through a broadband lambda probe corresponding to sectional line I-I in FIG. 2 in the area of the end of the probe body which projects into the exhaust gas stream,

[0024]FIG. 2 shows a top view corresponding to arrow II in FIG. 1 of the solid electrolyte layer between the reference electrode and the Nernst electrode, the contours of the layer applied to the above-mentioned layer by printing technology being shown for the reference air channel and

[0025]FIG. 3 shows a cross section of the end of the probe body on the reference air side parallel to the sectional plane of FIG. 1.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0026] According to FIGS. 1 and 3, the lambda probe shown has a body 1 which is designed as a ceramic laminate. The layers of the laminate are applied or stacked green. After subsequent sintering, which may take place after or simultaneously with pressing of the laminate, a hard ceramic body 1 is then produced.

[0027] In the example of FIGS. 1 and 3, a bottom layer 2 in the form of a thicker film of zirconium oxide is provided. An electrically insulating double layer 3 is provided above this layer, an electric resistance heater 4 and respective printed conductors for the electric power supply being embedded in it. Above this is a layer 5 produced by screen printing and structured, e.g., made of zirconium oxide paste. Within this layer, a reference air channel 6 is formed, an example of its outline being illustrated in FIG. 2 and explained in greater detail below. This reference air channel 6 has two end areas 6′ communicating with one another in the area of the sectional plane of FIG. 1.

[0028] Above layer 5 there is a solid electrolyte layer 7, e.g., in the form of a zirconium oxide film to which yttrium oxide has been added. At least in the area of end areas 6′ of reference air channel 6, a gas-permeable, laminar reference electrode 8 made of a porous platinum material is situated on the side of layer 7 facing reference air channel 6 or between layers 5 and 7 and is connected to a terminal contact on body 1, to be explained in greater detail below, by an adjoining laminar printed conductor 8′ (see FIG. 2)

[0029] Above solid electrolyte layer 8 there is a thin layer 9 which is structured by printing technology and is again made of zirconium oxide paste, for example. This layer 9 has a large recess which is arranged centrally with respect to an exhaust gas admission hole 10 passing through body 1 perpendicular to its layers. A porous material 12 is deposited within this recess, leaving an annular space 11 clear.

[0030] In the area of annular space 11, solid electrolyte layer 7 has a gas-permeable laminar Nernst electrode 13 made of a porous platinum material.

[0031] Another solid electrolyte layer 14, e.g., in the form of a zirconium oxide film containing yttrium oxide as an additive, is above layer 9, i.e. porous material 12. On its side facing annular space 11 as well as on its side facing away from annular space 11, this layer 14 has gas-permeable inner and outer pump electrodes 15 and 16 made of a platinum material that is porous in at least some areas, these electrodes 15 and 16 being shaped so that they at least essentially cover annular space 11 as seen in a top view of the layers of body 1. A gas-permeable protective layer 17 is also above layer 14.

[0032] In order for resistance heater 4 and various electrodes 8, 13, 15 and 16 to be electrically accessible from the outside, contact lugs (not shown) are situated on the reference airside end of body 1 which are connected to resistance heater 4 or electrodes 8, 13, 15 and 16 via through-contacts passing through one or more layers and adjoining conductors, which are usually produced by printing technology, running between adjacent layers.

[0033] According to FIG. 3, two through-contacts 18 passing through bottom layer 2 are provided for connecting electric resistance heater 4. In the example shown here, these through-contacts 18 have an annular or cylindrical shape.

[0034] In addition, a through-contact 19 also designed in an annular or cylindrical shape passing through the layers above layer 5 is provided for reference electrode 8. Contacts 18 and 19 may also be arranged coaxially.

[0035] To guarantee reliable electric insulation despite the small thickness of layer 5 and the small distance accordingly between the facing ends of contact 19 and coaxial contact 18, the electrically insulating material of layer 5 is drawn into through-contact 19 so that the bottom end of through-contact 19 in FIG. 3 is completely covered by electrically insulating material.

[0036] A through-contact 20 passing through layers 9, 14 and 17 is electrically connected to Nernst electrode 13 and to internal pump electrode 15. External pump electrode 16 is connected to a contact 21 via a printed conductor (not shown) passing through protective layer 17.

[0037] The production and structuring of layer 5 on layer 7 by printing technology are explained below with reference to FIG. 2.

[0038] Reference electrode 8 and respective printed conductor 8′ are first printed on film layer 7, typically by screen printing. Then a negative image of reference air channel 6 and its end pieces 6′ and any fan-like mouths 6″ optionally also present is applied with the material of layer 5. This is also performed by screen printing using appropriate masks, and it should be pointed out that it is possible to produce extremely fine filigree structures, optionally not coherent, in this way.

[0039] If the coating of solid electrolyte layer 7 is performed with the material of layer 5, then layer 7 is already stacked with the layers situated above layer 7 in FIGS. 1 and 3. In addition, through-contact 19 is also already positioned. Accordingly, the material of layer 5 may be drawn into the interior at through-contact 19, so that this interior space is covered by material 5′, at least in the lower area in FIG. 3, and thus is reliably insulated electrically at a later point in time with respect to through-contact 18, which is coaxial with through-contact 19. This guarantees in particular that no conducting connection may be created between coaxial through-contacts 18 and 19 due to fouling.

[0040] A positive image of reference air channel 6 and its end pieces 6′ and mouths 6″ may optionally also be printed onto layer 7 using a material which is dissolved or burned off or forms a porous, highly gas-permeable structure when body 1 is sintered.

[0041] It is also fundamentally possible to print layer 3 with the material of layer 5 in mirror image to layer 7 and optionally also to print it with the material provided for the positive image of reference air channel 6 and its parts 6′ and 6″. It is possible in this way to produce layer 5 with a greater thickness.

[0042] The lambda probe described above functions as follows:

[0043] The end of body 1 having exhaust gas admission hole 10 is situated in the exhaust gas stream, i.e. in an area communicating with the exhaust gas stream of an internal combustion engine, while the other end of body 1 is acted upon by reference air, usually atmospheric air.

[0044] Reference air reaches end pieces 6′ of the reference air channel through reference air channel 6, i.e. its mouths 6″. Exhaust gas goes through exhaust gas admission hole 10 to porous material 12 through which the exhaust gas diffuses into annular space 11.

[0045] When body 1 is heated sufficiently by electric resistance heater 4, an electric voltage may be picked up between reference electrode 8 and Nernst electrode 13 and thus between through-contacts 19 and 20, the size of this voltage depending on the oxygen partial pressures within end pieces 6′ of the reference air channel and within annular space 11. This makes use of the effect whereby the platinum material of aforementioned electrodes 8 and 13 promotes or permits the formation of oxygen ions, the result being that an ion diffusion which depends on the concentration of oxygen ions on electrodes 8 and 13 occurs in solid electrolyte layer 7, leading to an electric potential difference between electrodes 8 and 13.

[0046] The oxygen partial pressure in annular space 11 may be controlled by applying an external electric voltage having a controllable polarity between pump electrodes 15 and 16. The corresponding voltage source is connected to through-contact 20, i.e. to contact 21. Here again, this makes use of the effect whereby the platinum material of electrodes 15 and 16 leads to the formation of oxygen ions, and then an oxygen ion current having an amperage and direction which depend on the electric voltage and polarity and which diffuses through solid electrolyte layer 14 is created due to the external electric voltage between electrodes 15 and 16. An electric current is detectable between pump electrodes 15 and 16.

[0047] Then the electric voltage and thus also the electric current between pump electrodes 15 and 16 are controlled by a regulator so that the electric voltage available between reference electrode 8 and Nernst electrode 13 always corresponds to a fixed setpoint. The electric current available between pump electrodes 15 and 16 is thus a measure of the oxygen content of the exhaust gases relative to the reference air.

[0048] If external pump electrode 16 is at a positive electrical potential with respect to internal pump electrode 15, then the prevailing operating conditions have λ>1. When the polarity is reversed, the prevailing operating conditions have λ<1, the measure of the electric current detectable between electrodes 15 and 16 correlating with the size of λ.

[0049] The values of λ may be detected in a large value range.

[0050] In the case of narrow-band lambda probes mentioned in the preamble, external protective layer 17 is above Nernst electrode 13, i.e., layers 9 and 14 and pump electrodes 15 and 16 are omitted in comparison with the diagrams in FIGS. 1 and 3. The electric voltage detectable between electrodes 8 and 13 is then a measure of the oxygen partial pressure of the exhaust gases.

[0051] Depending on the design of the lambda probes for narrow-band or broadband measurement, reference air channel 6 together with its parts 6′ and 6″ may be produced in the manner described above by deposition of structured layer 5 by printing technology. 

What is claimed is:
 1. A gas sensor, in particular a lambda probe, having a body (1) designed as a sintered ceramic laminate and therein a reference air channel (6) arranged inside a layer (5) of the laminate, an electric resistance heater (4) embedded in an electrically insulated manner in the laminate is provided on one side thereof and an electrode arrangement (8, 13; 15, 16) is provided on its other side which has at least one reference electrode (8) which is arranged inside a bordering wall of the reference air channel and is gas-permeable in at least some areas and also has a Nernst electrode (13) which may be acted upon by the gas to be sensed and is also designed to be gas-permeable in at least some areas and is separated from the reference electrode (8) by a solid electrolyte layer (7) that is conductive and permeable for ions, in particular oxygen ions, wherein the reference air channel (6) is designed in a structured layer or layer arrangement (5) produced by printing technology, in particular by screen printing.
 2. The gas sensor according to claim 1, wherein at the electric resistance heater (4), in the vicinity of the reference air channel (6) or its parts (6′), large surface areas in comparison to the reference air channel, of the layer (5) structured by printing technology are provided for the heat-conducting coupling of the resistance heater to the electrolyte layer (7) between the reference electrode and the Nernst electrode (8, 13).
 3. The gas sensor according to claim 1 or 2, wherein the reference air channel (6) is filled with a porous material having good gas-permeable properties.
 4. The gas sensor according to one of claims 1 through 3, wherein the reference air channel (6) is situated outside (as seen in a top view of the layer planes of the laminate) an admission opening (10), for the gas to be sensed, perpendicular to the layer planes and arranged in the body (1).
 5. The gas sensor according to one of claims 1 through 4, wherein the reference air channel (6) is situated outside through-contacts (18 through 20) as seen in a top view of the layer planes of the laminate, these through-contacts connecting electrodes (8, 13, 15, 16) or printed conductors electrically connected to the electrodes to contacts situated on the outside of the body (1) or they are designed as such.
 6. The gas sensor according to one of claims 1 through 5, wherein the reference air channel (6) opens outward on at least one long side of the layer (5) having the channel (mouths 6″).
 7. The gas sensor according to one of claims 1 through 6, wherein the reference air channel (6) has mouths (6″) arranged in a fan-like pattern for admission of the reference air.
 8. The gas sensor according to one of claims 1 through 7, wherein through-contacts (19) passing through one or more layers are provided with ring-shaped cross sections having an electrically insulating coating (5′) on the inside as seen in a top view of the layer planes.
 9. The gas sensor according to claim 8, wherein the layer (5) having the reference air channel is made of the same material as the coating (5′) of the through-contact (19).
 10. A method of producing a gas sensor according to one of claims 1 through 9, wherein a negative pattern of the reference air channel (6) is printed with a material, such as zirconium oxide paste that hardens under heat, on the side of a layer (3) carrying or enclosing the resistance heater (4), on the side facing the reference channel (6) and/or on the side of the solid electrolyte layer (7) arranged between the reference electrode and the Nernst electrode (8, 13), that side carrying the reference electrode (8).
 11. The method according to claim 10, wherein a positive pattern of the reference air channel (6) is also printed using a material that burns off under heat or develops into a porous, having good gas-permeable properties.
 12. The method according to claim 10 or 11, wherein the material is applied by screen printing. 